The conventional extendable/contractible type ball point pens and mechanical pencils write through the exposure of the writing medium from the pointed end of a cap. An eraser, or the like, with which the writing is erased, is attached at the end opposite to the cap.
In such cases, the eraser is exposed through an operation entirely separate from that of the extension and contraction of the writing medium. An example is the removal of an eraser cap to expose an eraser. Removing the eraser cap is cumbersome, and there is a possibility that the eraser cap will be lost.
Also, when the conventional extendable/contractible ballpoint pens are taken out of use, a sliding aglet is pressed to retract the ends of a shaft so as to push the writing tip of the ballpoint refill into a concealed position. When in use, the sliding aglet is pressed so as to cause the ends of the shaft to expand lengthwise and push the writing tip of the ink refill to the exposed position.
The conventional extendable/contractible type ballpoint pens require a high degree of machining accuracy, and the cost of assembly is high due to the complexity of the moving parts. Accordingly, there is a need to provide a writing instrument that eliminates the aforementioned drawbacks and provides further advantages.